app: more accurate labels for "Visible" actions.

There were 4 actions displaying as "Visible" only: channels-visible,
drawable-visible, layers-visible and vectors-visible. This was not very
useful to differentiate them (for instance in action search). Just make
clearer labels.

app/actions/channels-actions.c

@@ -110,7 +110,7 @@ static const GimpActionEntry channels_actions[] =
 static const GimpToggleActionEntry channels_toggle_actions[] =
 {
   { "channels-visible", GIMP_ICON_VISIBLE,
-    NC_("channels-action", "_Visible"), NULL, NULL,
+    NC_("channels-action", "Toggle channel _visibility"), NULL, NULL,
     G_CALLBACK (channels_visible_cmd_callback),
     FALSE,
     GIMP_HELP_CHANNEL_VISIBLE },